1. Field of the Invention
The present invention relates to resonators, filters, duplexers, composite filter apparatuses, and transmitting and receiving apparatuses that are used in RF circuits of communication equipment, and to communication apparatuses that use the resonators, the filters, the duplexers, the composite filter apparatuses, and the transmitting and receiving apparatuses.
2. Description of the Related Art
Generally, resonators such as dielectric resonators provided with electrodes on dielectrics are used for microwave communication. The dielectric resonators are, for example, microstrip resonators and dielectric coaxial resonators.
Along with the improved performance of communication apparatuses, low-loss characteristics of resonators are becoming more important. Dielectric resonators in which superconductors are used as electrodes have low conductor loss. For such resonators, however, in order to maintain low-loss characteristics, the temperature must always be lower than the critical temperature at which the electrodes become superconductive. Thus, the resonators must always be cooled by refrigerators. For example, a failure to cool the resonators due to a malfunction of the refrigerator, however, causes the temperature of the resonator electrodes to exceed the critical temperature. Thus, the conductance of the superconductor becomes much lower than that of metals, which are normally used as electrode materials, and also, the resistance of the superconductor is increased. Therefore, the conductor loss of the resonators is increased.
Microstrip dielectric resonators for solving such problems are disclosed in Japanese Unexamined Patent Application Publication Nos. 6-37513 and 6-37514.
Referring to FIGS. 17A, 17B, and 17C, in these microstrip dielectric resonators, a resonance electrode 2 with a predetermined width and length is formed on a first main surface of a dielectric substrate 1 and a ground electrode 3 is formed over an entire second main surface of the dielectric substrate 1. The resonance electrode 2 comprises a superconducting film 21 and a metal film 22 deposited in that order, and the ground electrode 3 comprises a superconducting film 31 and a metal film 32 deposited in that order. In such dielectric resonators, the superconducting film 21 operates as a main resonance electrode in low-temperature operation below the critical temperature, and the metal film 22 operates as a main resonance electrode in high-temperature operation at or above the critical temperature. Accordingly, reduction of conductor loss in the normal temperature range can be suppressed.
Such conventional low-loss dielectric resonators, however, have the following problems.
The surface reactance of superconductors for RF signals significantly differs between the superconductive state in low-temperature operation below the critical temperature and the non-superconductive state in high-temperature operation at or above the critical temperature. Thus, the resonance frequency of the resonators significantly differs between the superconductive state and the non-superconductive state, as shown in FIG. 18.
FIG. 18 is a graph showing the temperature characteristics of the resonance frequency of a dielectric resonator using a layered electrode comprising a superconductor and a metal.
As shown in FIG. 18, although the resonance frequency gradually decreases in both the superconductive state and non-superconductive state as the temperature increases, the resonance frequency significantly drops when the state changes from the superconductive state to the non-superconductive state. As described above, the resonance frequency is completely changed at the critical temperature. If, for example, a band pass filter is formed by such a resonator, the width of the pass band varies with temperature, and the transmission characteristics are thus disadvantageously dependent on temperature.